On the existence of integer relative Heffter arrays (1910.09921v2)

Abstract: Let $v=2ms+t$ be a positive integer, where $t$ divides $2ms$, and let $J$ be the subgroup of order $t$ of the cyclic group $\mathbb{Z}_v$. An integer Heffter array $H_t(m,n;s,k)$ over $\mathbb{Z}_v$ relative to $J$ is an $m\times n$ partially filled array with elements in $\mathbb{Z}_v$ such that: (a) each row contains $s$ filled cells and each column contains $k$ filled cells; (b) for every $x\in \mathbb{Z}_v \setminus J$, either $x$ or $-x$ appears in the array; (c) the elements in every row and column, viewed as integers in $\pm\left{ 1, \ldots, \left\lfloor \frac{v}{2}\right\rfloor \right}$, sum to $0$ in $\mathbb{Z}$. In this paper we study the existence of an integer $H_t(m,n;s,k)$ when $s$ and $k$ are both even, proving the following results. Suppose that $4\leq s\leq n$ and $4\leq k \leq m$ are such that $ms=nk$. Let $t$ be a divisor of $2ms$. (a) If $s,k \equiv 0 \pmod 4$, there exists an integer $H_t(m,n;s,k)$. (b) If $s\equiv 2\pmod 4$ and $k\equiv 0 \pmod 4$, there exists an integer $H_t(m,n;s,k)$ if and only if $m$ is even. (c) If $s\equiv 0\pmod 4$ and $k\equiv 2 \pmod 4$, then there exists an integer $H_t(m,n;s,k)$ if and only if $n$ is even. (d) Suppose that $m$ and $n$ are both even. If $s,k\equiv 2 \pmod 4$, then there exists an integer $H_t(m,n;s,k)$.